Observations and analysis of two wet-snow avalanche cycles

Wet-snow avalanches threaten mountain communities and communication lines. Their formation as well as the snowpack processes leading to wet-snow instability are poorly understood. Forecasting wet-snow avalanches is a great challenge and poses great difficulties for local authorities. Better knowledge about the processes leading to wet-snow instabilities is therefore very important. During the winters of 2007-2008 and 2008-2009 two distinct wet-snow avalanche cycles occurred in the surroundings of Davos, Switzerland. We analyzed meteorological data, in-situ snowpack informa- tion and mapped avalanche extent. In addition, the snow cover model SNOWPACK was used to fill the gap where snowpack data, such as volumetric water or snow temperature, were not available. The analysis focused on the causes of instability: loading and/or weakening due to water infiltration. The full energy balance was calculated using meteorological data and extrapolated to the investigation area using the model ALPINE3D. Both avalanche cycles occurred in a short period of time. Precipita- tion amounts and the type of precipitation, i.e. rain or snow, played an important role during the first avalanche cycle, while terrain parameters such as aspect and slope angle combined with liquid water infiltration patterns were crucial during the second wet-snow avalanche cycle. Although different me- teorological conditions prevailed during these two avalanche cycles, it appears that wet-snow instabili- ties were mostly influenced by snow stratigraphy, rapid increase in air temperature and water infiltra- tion patterns.